The invention is related to an apparatus and method for periodically producing an adaptable pressure waveform in a pneumatic sleeve applied to a limb of a human patient in order to help prevent deep vein thrombosis (DVT) or to treat lymphedema in the patient.
Limb compression systems of the prior art apply and release pressure on a patient""s extremity to augment venous blood flow and help prevent deep vein thrombosis (DVT) or to treat lymphedema. Limb compression systems of the prior art typically include: a source of pressurized gas; one or more pneumatic sleeves for attaching to one or both of the lower limbs of a patient; and an instrument connected to the source of pressurized gas and connected to the sleeves by means of pneumatic tubing, for controlling the inflation and deflation of the sleeves and their periods of inflation and deflation. In U.S. Pat. No. 3,892,229 Taylor et al. describe an early example of one general type of limb compression system of the prior art known as an intermittent limb compression system; such systems apply pressure intermittently to each limb by inflating and deflating a single-bladder sleeve attached to the limb. In U.S. Pat. No. 4,013,069 Hasty describes an example of a second general type of limb compression system of the prior art, known as a sequential limb compression system; such systems apply pressure sequentially along the length of the limb by means of a multiple-bladder sleeve or multiple sleeves attached to the same limb which are inflated and deflated at different times. Certain intermittent and sequential limb compression systems of the prior art are designed to inflate and are deflate sleeves on both limbs either simultaneously or alternately, while others are designed for use on one limb only.
The primary purpose of most of the limb compression systems of the prior art is to prevent or, reduce the risk of DVT. Such limb compression systems are used to minimize venous stasis during and immediately following surgery, as well as during long periods of immobility. DVT may lead to pulmonary embolism (PE), a serious hazard for surgical and trauma patients. For example, patients over forty years of age who are undergoing hip or knee surgery, or major abdominal surgery, are at particular risk of DVT. When DVT leads to PE, this complication can result in death, with an estimated 200,000 such deaths occurring in the United States annually. To help prevent DVT and thus PE, the use of pneumatic limb compression systems of both intermittent and sequential types, used either alone or combined with anticoagulant drug therapy, have been developed in the prior art and are commonly used at present.
A purpose of other limb compression systems of the prior art is to treat chronic edema, including lymphedema. Lymphedema refers to the condition of fluid accumulation in a limb. Secondary lymphedema can be a result of trauma or surgical complications. Limb compression therapy using limb compression systems of the prior art has been demonstrated to be of significant value in treating lymphedema.
Systems of the prior art have not been capable of producing a desired pressure waveform in a pneumatic sleeve attached to a limb. This is a significant limitation, as the inventors of the present invention have inferred from the recent clinical literature that applied pressure waveforms having differing shapes produce significantly different changes to venous blood flow. In the clinical literature, the use of a wide range of devices and non-standardized techniques by clinicians to indicate changes in venous flow and venous stasis, either subjectively or quantitatively, has been reported. For example, devices employing Doppler ultrasound, photo-plethysmography, impedance plethysmography, contrast venography, oximetry and de-oximetry have all been used for such purposes in the prior art. Such changes, when detected, then may or may hot have been taken into consideration in the manual adjustment of prior-art systems. For example, Tumey et al. in U.S. Pat. No. 5,443,440 describe apparatus including a sensor for determining whether patients have venous blood flow problems prior to setting parameters and use. However, a significant limitation of many prior-art limb compression systems is that such systems have not incorporated a standardized physiologic transducer and measurement algorithm which provides an indication of the change in venous blood flow produced as a result of the application of a pressure waveform to by means of the sleeve of the system. As a result, these prior-art systems cannot automatically adapt or change the pressure waveform applied to the limb, nor can they permit an operator to manually adapt or change the pressure waveform, in response to changes in venous blood flow, in order to improve the effectiveness of the therapy.
In many sequential limb compression systems of the prior art, such as the one described by Hasty in U.S. Pat. No. 4,013,069, elapsed times are pre-set to initiate the sequential pressurization of each of the multiple-chamber sleeves, or each of the multiple sleeves. This has been a significant limitation and has produced a sub-optimal augmentation of venous blood flow by such sequential limb compression systems, but has been necessary because these prior-art systems have not been capable of producing desired pressure waveforms in multiple-bladder sleeves and multiple sleeves, and have thus not been capable of using a selected parameter of the pressure waveform in one sleeve or bladder of a multiple-bladder sleeve to trigger the pressurization of another sleeve or bladder using the desired pressure waveform for that sleeve or bladder.
Many limb compression systems of the prior art are not capable of producing a desired pressure waveform in a pneumatic sleeve attached to a limb either because they do not directly measure the pneumatic pressure in the sleeve at any instant, or because they do not generate a signal indicative of the pressure suitable for permitting a feedback control system to produce the desired pressure waveform. In the prior art, for example, pressure gauges have been connected to inflatable bladders to provide visual indications of bladder pressure to operators, but such apparatus did not generate a signal suitable for controlling the production of a waveform and the apparatus was considered to be expensive, inconvenient and unnecessary.
Some limb compression systems of the prior art attempt to prevent hazardous over-pressurization by limiting the maximum pressure level produced in the sleeve without actually displaying or measuring the sleeve pressure. For example, in U.S. Pat. No. 4,841,956 Gardner et al. describe a limb compression system in which sleeve pressure is not measured, but in which the peak pressure level is limited by limiting the time period during which inflating gas flows into the sleeve. In such a system the maximum pressure actually produced in the sleeve is dependent on variables such as the flow resistance of the tubing, the design and pneumatic volume of the sleeve, and the pressure of the gas during the inflating time period. Other systems, such as that of Arkans in U.S. Pat. No. 4,396,010, use a preset pressure switch in the instrument to limit the maximum pneumatic pressure level.
In a limb compression system described by Cariapa et al. in U.S. Pat. No. 5,437,610, a pressure sensor is connected to a fluid-filled bladder within a pneumatic sleeve, but the sensor/bladder combination is adapted to measure the static pressure of the limb against the uninflated sleeve, and could not be used or adapted to produce any one of a wide range of desired pneumatic pressure waveforms in the sleeve.
Some limb compression systems known in the prior art attempt to estimate sleeve pressure in an inexpensive and convenient manner, based on a variety of apparatus and methods. These systems do not measure pressure directly in the pneumatic sleeve applied to the limb but instead estimate sleeve pressure indirectly and remotely from the sleeve. For example, in U.S. Pat. No. 5,031,604 Dye describes a system in which sleeve pressure is estimated by measuring pneumatic pressure near the instrument end of the tubing connecting the instrument to the sleeve. As another example, Arkans in U.S. Pat. No. 4,375,217 describes a system in which the static pressure in the sleeve is estimated at a location on the tubing between the instrument and the sleeve. All such apparatus and methods which estimate sleeve pressure by measuring a pneumatic pressure remotely from the sleeve suffer from a significant disadvantage, which makes them unsuitable for incorporation into an instrument for producing a desired pressure waveform in the sleeve: the accuracy of the estimates of pressure made by such systems is significantly affected by variations in the length and flow resistance of the tubing attached to the sleeve, and by variations in sleeve design, sleeve inflation volume and sleeve application technique. For example, the inventors of the present invention have determined that variables related to the design and size of the sleeve, as well as the snugness of application of the sleeve, can result in discrepancies at any instant of well over 50 percent between the remotely estimated sleeve pressure and the actual pressure in the sleeve. As a separate consideration regarding the flow resistance of the tubing employed in prior-art systems which measure pressure in this manner, it has been necessary to locate such systems close to the patient to minimize flow resistance in the tubing, resulting in unnecessary noise and clutter around the patient.
Other systems known in the prior art interrupt the flow of gas in the tubing in an effort to estimate sleeve pressure by measuring pneumatic pressure at the instrument end of the tubing under zero-flow conditions. One such system is the Jobst Athrombic Pump System 2500 (Jobst Institute Inc., Charlotte N.C.). However, estimates of sleeve pressure made in this manner cannot practically be incorporated into limb compression systems for producing pressure waveforms having large amplitudes and short cycle periods. Also, more generally, such systems suffer from the disadvantage that pressure estimates are available discontinuously and are not suitable for real-time control of the pressure in the sleeve to produce a desired pressure waveform.
In the prior art, incorporation of a force sensor to measure the force applied by a sleeve to a limb has been described by Tumey et al. in U.S. Pat. No. 5,443,440. Also, the use of separate measurement apparatus for measuring the pressure applied by a sleeve to a limb has been described by Arkans in U.S. Pat. No. 4,331,133, wherein a separate measurement cuff is placed between the sleeve and the limb and the pressure applied by the sleeve is estimated indirectly. Both the above-referenced force sensor of Tumey et al. and separate measurement apparatus of Arkans have several disadvantages which make them unsuitable for incorporation into a system for periodically applying a desired pressure waveform to a limb: calibration of the force sensor/measurement cuff is difficult, time-consuming and error-prone; significant errors can arise during use due to use-related changes in the interface between force sensor/measurement cuff and the sleeve, or between the force sensor/measurement cuff and the limb; and minor anomalies such as wrinkling or folding of the sleeve or cuff surface when inflated can produce significant anomalies in measured force/pressure.
Because of errors and limitations associated with estimation of the pressure applied by a sleeve to a limb, prior-art systems have not had the capability of accurately producing a desired pressure waveform in combination with sleeves having differing designs and varying pneumatic volumes, or when sleeve application techniques vary and the resulting sleeve snugness varies, or when sleeves are applied to limbs of differing sizes, shapes and tissue characteristics. As a result, clinical staff using such prior-art systems have very inaccurate and limited knowledge of what pressure waveforms have actually being applied to the patient, relative to what was prescribed.
In addition to their limitations related to the delivery of desired pressure waveforms for pneumatic compression therapy, an important limitation of all systems known in the prior art is that they do not allow clinically important characteristics of pressure waveforms to be changed independently of each other. For example, characteristics of pressure waveforms that have been shown in the clinical literature to significantly affect the augmentation of venous blood flow and thus the incidence of DVT, PE and patient outcomes include: the maximum pressure, the rate of pressure increase to the maximum pressure, the time at which the maximum pressure occurs and the duration after the maximum pressure that the pressure remains above a threshold. No systems known in the prior art allow each of these characteristics to be changed independently of the other. For example, the apparatus described by Tumey et al. in U.S. Pat. No. 5,443,440 allows a user to change one characteristic, the preset maximum pressure value, but changing the maximum pressure value also necessarily changes the rate of pressure increase leading up to the maximum pressure and other clinically important characteristics of the pressure waveform inherently produced by the apparatus of Tumey at al. Tumey et al. is thus representative of prior art systems wherein an inherent pressure waveform is produced by a combination of pneumatic, electronic, mechanical and other components of the apparatus as well as user settings, and wherein any attempt to change one clinically significant aspect of the inherent pressure waveform changes other clinically important characteristics of the delivered pressure waveform. This is an important limitation of all such prior-art systems because the ability to change such characteristics of pressure waveforms independently is desirable in order to optimize pneumatic compression therapy and thus improve patient outcomes.
In U.S. Pat. No. 5,443,440 Tumey et al. further describe a limb compression system capable of creating and storing the time, date and duration of each use of the system for subsequent transmission to a physician""s computer. However, sequential and intermittent limb compression systems known in the prior art do not record parameters related to the periodic application of a desired pressure waveform, such as any differences between the actual shape of the pressure waveform produced in the pneumatic sleeve and the shape of a desired reference pressure waveform, the time and duration during which the waveform was periodically applied, and the number of cycles of the waveform which were applied. Additionally, limb compression systems known in the prior art do not subsequently produce the recorded values of these parameters for use by physicians in determining the extent to which the desired pressure waveform was actually applied, for use by third-party payors in reimbursing for therapy actually provided, and for use in patient outcome studies where variations in these parameters of therapy are thought to be related to variations in patient outcomes, leading to optimization of waveform-related parameters and thus improved therapy.
The invention is directed to apparatus for applying an adaptable pressure waveform to a patient""s limb for augmenting venous blood flow in the limb, comprising: an inflatable sleeve for positioning onto a limb to apply a pressure to the limb beneath the sleeve when inflated with gas; pressure transducing means for sensing the pressure of gas in the sleeve; pressure waveform application means for controlling the supply of pressurized gas to the sleeve so that the sensed pressure in the sleeve is maintained near a varying pressure indicated by a reference pressure waveform; and waveform register means for producing the reference pressure waveform during a cycle time period wherein the reference pressure waveform includes a maximum pressure at a time within the cycle time period and a rate of pressure increase in an interval leading to the maximum pressure and wherein the waveform register means includes waveform adaptation means for allowing the rate of pressure increase to be changed from a first rate to a second rate without changing the maximum pressure at the time. The waveform adaptation means of the invention also allows the maximum pressure of the reference pressure waveform to be changed from a first level to a second level at the time within the cycle time period without changing the rate of pressure increase to the maximum pressure.
The invention may include provision for automatic adaptation of the reference pressure waveform and may also allow for adaptation of the reference pressure waveform by an operator.
Alarm means may be included for producing an alarm signal near an alarm time when the difference between the pressure indicated by the level of the sleeve pressure signal and the pressure indicated by the reference pressure waveform signal is greater than a predetermined pressure difference. The apparatus may also include therapy register means for enabling an operator to determine at a time subsequent to the alarm time the sleeve pressure and the reference waveform pressure indicated by the levels of the sleeve pressure signal and the reference pressure waveform signal recorded near the alarm time.
Advantageously, the inflatable sleeve of the invention includes a first sleeve connector means communicating pneumatically with the inflatable sleeve and a second sleeve connector means communicating pneumatically with the inflatable sleeve, and the first sleeve connector means does not communicate pneumatically with the second sleeve connector means except through the sleeve.